Subcellular visualization and quantification of cyanotoxin synthesis in cyanobacteria reveals distinct compartmentation.

The bloom-forming cyanobacteria Microcystis and Planktothrix produce cyanotoxins such as microcystins (MCs) and anabaenopeptins (APs) through nonribosomal peptide synthesis (NRPS). Close inspection of their synthesis can aid in understanding the biological function of such cyanotoxins. MCs and APs were visualized using chemoselective labeling, and a heterogeneous distribution of fluorescence signals was observed during both time-lapse signal build-up and decline experiments. The formation of rather distinct entities was quantified using advanced imaging analysis, defining modeled entities (ME) with varying intensities but rather stable volumes, numbers and localization. The origin of MC synthesis was mostly in the cytoplasm of unicellular Microcystis and was confirmed through immunolabeling. In contrast, for APs in filamentous Planktothrix, the origin was localized in the membrane region near the septa between filament cells. Our data suggest distinct cyanotoxin biosynthesis centers in cells, differing in their localization between unicellular and multicellular cyanobacteria.